1. Field of the Invention
The present invention relates generally to diffractive optical systems. The present invention relates more particularly to diffractive MEMs optical systems used for communication systems or projection displays.
2. Description of the Background Art
Dynamic gain equalizer and other subsystems for optical telecommunication systems may be formed using one or more arrays of light-modulating pixels. The light-modulating pixels may comprise, for example, GRATING LIGHT VALVE (GLV) pixels. In Dynamic gain equalizer, the optical channels in an optical fiber are dispersed over the pixels of the GLV which attenuates the channels in a controlled manner such that at the output of the device all channels have equal power.
In addition to telecommunication systems, such light-modulating pixels may also be used to form a two-dimensional projection. Publications describing GLV devices and their application to display systems include: “The Grating Light Valve: Revolutionizing Display Technology,” by D. M. Bloom, Projection Displays III Symposium, SPIE Proceedings, Volume 3013, San Jose, Calif., February 1997; “Grating Light Valve Technology: Update and Novel Applications,” by D. T. Amm and R. W. Corrigan of Silicon Light Machines in Sunnyvale, Calif., a paper presented at the Society for Information Display Symposium, May 19, 1998, Anaheim, Calif.; “Optical Performance of the Grating Light Valve Technology,” David T. Amm and Robert W. Corrigan of Silicon Light Machines, a paper presented at Photonics West-Electronics Imaging, 1999; “An Alternative Architecture for High Performance Display,” R. W. Corrigan, B. R. Lang, D. A. LeHoty, and P. A. Alioshin of Silicon Light Machines, a paper presented at the 141st SMPTE Technical Conference and Exhibition, Nov. 20, 1990, New York, N.Y.; and U.S. Pat. No. 6,215,579, entitled “Method and Apparatus for Modulating an Incident Light Beam for Forming a Two-Dimensional Image,” and assigned at issuance to Silicon Light Machines. Each of the above-mentioned publications is hereby incorporated by reference in its entirety. In such display systems, the linear array modulates an incident light beam to display pixels along a column (or, alternatively, a row) of the two-dimensional (2D) image. A scanning system is used to move the column across the screen such that each light-modulating pixel is able to generate a row of the 2D image. In this way, the entire 2D image is displayed.
FIG. 1 is a diagram depicting the reflective and diffractive operational states of a conventional grating light valve (GLV) element. The left side of the diagram depicts the reflective state, while the right side of the diagram depicts the diffractive state.
In the example illustrated in FIG. 1, the substrate may comprise a silicon substrate with oxide (for example, about 5000 angstroms thick) overlaying the silicon, and tungsten (for example, about 1000 angstroms thick) overlaying the oxide. The reflective members lie above the tungsten with an air space there between. For example, three pairs of reflective members (i.e. six members) are shown. The reflective members may, for example, comprise reflective ribbons comprising nitride (for example, about 1000 angstroms thick) with a reflective layer of aluminum (for example, about 500 angstroms thick) on the nitride. Incident light is beamed onto the reflective members. The incident light beam may be at a perpendicular angle to the plane of the substrate.
In the reflective state (left side), all the reflective members are in the same plane, and the incident light is reflected from the surfaces of the members. This reflective state may be called the zero attenuation state because it may be used to reflect all the light back (minus some system loss) back on the same path as the incident beam.
In the diffractive state (right side), alternate ones of the reflective members are deflected downward. This results in the diffraction of the incident light in a direction that is at an angle to the path of the incident light. This diffractive state may be called the attenuation state because it may be used to attenuate the light reflected back on the same path as the incident beam. As discussed further below, the optical response of the element depends on the amount of downward deflection of the alternate members.
As depicted in FIG. 2, the conventional GLV element may include pairs of reflective ribbons, each pair having one fixed and one movable ribbon.
FIG. 3 is a diagram depicting deflections of reflective members for a conventional GLV element in a diffractive state. The conventional GLV element comprises a plurality of reflective members 302. In the example illustrated, the GLV element includes three pairs of reflective members 302 (i.e. six of them).
In the diffractive state, the reflective members are controllably arranged in an alternating configuration at two heights (304 and 306) with respect to the grating plane 308. A first height 304 may be positioned farther from the grating plane 308, while the second height 306 may be displaced closer to the grating plane 308. For example, as shown in FIG. 2, the reflective members at the first height may comprise fixed reflective ribbons, while the reflective members at the second height may comprise movable reflective ribbons.
The grating plane 308 is a theoretical plane that corresponds to the plane on or about which the reflective members are aligned. As illustrated in FIG. 3, the incident light beam 310 impinges upon the element at an angle perpendicular to the grating plane 308. Diffracted light 312 travels away from the element.
The difference between first and second heights may be defined as the deflection distance γ. The amount of the deflection γ may be varied to control the amount of incident light reflected from the element. When γ is near zero, the element would be near a maximally reflective state. When γ is near λ/4, where λ is the wavelength of the incident light, the element would be near a maximally diffractive state.
With some simplification, when γ is near λ/4, the diffracted light 312 may be considered as traveling outward at an angle β with respect to the incident light 310. Making some approximations, β=arcsin (λ/P), where P is the pitch of the pairs of reflective members (i.e. the horizontal distance between the beginning of one pair and the beginning of the next pair) when the incident light beam 310 impinges upon the element at an angle perpendicular to the grating plane 308.
FIG. 4 is a graph illustrating a non-linear optical response for a conventional GLV element as a function of amount of deflection, γ. The graph shows intensity of light (in arbitrary units) versus γ (from 0 to λ/4). As the deflection (γ) increases from 0 to λ/4 the intensity of light decreases. The decrease in intensity of light with deflection is highly non-linear and the rate (slope) of decrease in the intensity of light increases with the increase in deflection (γ) increases from 0 to λ/4. If the optical response were linear, then the graph would show a straight line. However, as shown in FIG. 4, the optical response is disadvantageously non-linear.